I have IBM System x3650 M2
and it has only SAS drives attached
It has 7 PCI-e
empty slots.
I want to know what type of card I need which I can put in PCI-E so that I can join 2 SATA drives of higher capacity like 4TB or 3TB
None.
You need no extra card. A SAS connector accept both SAS and SATA drives.
Should all SAS connectors already be filled then you have the choice between:
(SATA drives on port multipliers also work but are NOT recommended since SATA can fail non-gracefully and block access to all drives on the port multiplier.)
You need a SATA RAID controller that can handle those sized drives.